The DNS protocol (Cont.)
A DNS client queries its pre-configured (local) DNS server for some DNS info. If the local DNS has this info in its cache, it sends the info back to the client.
Otherwise, the DNS server queries one of the root servers (whose addresses is has in its static configuration) for the DNS of the relevant TOP level domain.
Given the address of the relevant DNS server, the local DNS server sends a query to the secondary DNS server. If it has the answer, it will pass it back, otherwise, it passes the address of the DNS server responsible for the relevant 3rd-level domain.
All this data exchange is done using the UDP protocol.